1. Field of the Invention
The present invention relates to a coordinate detection apparatus and method, which detect the coordinates of a light-shielded position, and a computer program.
2. Description of the Related Art
A coordinate input apparatus which controls a connected computer or writes characters, figures, and the like by inputting coordinates by pointing to a coordinate input surface using a pointer (e.g., a dedicated input pen, finger, or the like) is known.
Conventionally, various types of such coordinate input apparatuses are proposed or commercially available as touch panels to manipulate terminals such as personal computers or the like on the screen.
Various coordinate input methods such as a method using a resistive film, that using ultrasonic waves, and the like are available. For example, Japanese Patent Laid-Open No. 2005-078433 proposes the following coordinate input apparatus. That is, a light emitting unit is provided to the tip portion of a pointer as a dedicated writing material, a light source of the pointer emits light by a touch input operation by that pointer, and that light is detected by light receiving units provided to the corners around a coordinate input effective region, thereby calculating a touch input position of the writing material.
As an apparatus using light, for example, U.S. Pat. No. 4,507,557 is available. U.S. Pat. No. 4,507,557 discloses the following arrangement. That is, a retroreflecting sheet is arranged outside a coordinate input effective region, and illumination units for illuminating light and light receiving units for receiving light, which are arranged at the corners of the coordinate input effective region, detect angles between the light receiving units and an object such as a finger or the like, which shields light, within the coordinate input effective region. Based on the detection result, the pointed position of the object is decided.
U.S. Pat. Nos. 6,429,856, 6,570,103, and the like disclose coordinate input apparatuses, which detect the coordinates of a part (light-shielded part) where retroreflecting light is shielded by arranging retroreflecting members around a coordinate input effective region.
In these apparatuses, for example, in U.S. Pat. No. 6,429,856, the peak of a light-shielded part by an object, which is received by light receiving units, is detected by waveform processing arithmetic operations such as differentiation and the like to detect the angles of the light-shielded part with respect to the light receiving units, and the coordinates of the object are calculated based on the detection result. U.S. Pat. No. 6,570,103 discloses an arrangement in which one end and the other end of a light-shielded part are detected by comparing with a specific level pattern, and the center of these coordinates is detected.
A method that calculates coordinates by detecting a light-shielded position like U.S. Pat. Nos. 4,507,557, 6,429,856, and 6,570,103 will be referred to as a light-shielding method hereinafter.
Japanese Patent Laid-Open No. 2000-347798 discloses a coordinate input apparatus in which a retroreflecting sheet is provided outside a coordinate input surface of this type and reflects light coming from illumination units that illuminate light, and light receiving units detect light amount distributions, thereby detecting the light-shielded position. In the arrangement described in this reference, the coordinate input apparatus comprises a detection unit which detects dust inside a coordinate detection effective region or contamination of the retroreflecting sheet, and outputs an input error signal to prevent malfunctions when this detection unit detects dust or contamination.
Japanese Patent Laid-Open No. 2001-043019 discloses an arrangement which has an arithmetic unit that diagnoses whether or not a coordinate detection result is normal, and in which this arithmetic unit comprises a storage unit for storing reference data used in self diagnosis, and executes self diagnosis by comparing the reference data with self diagnosis data based on reflected light.
The coordinate input apparatus that adopts the light-shielding method acquires base data in a state without any illumination of illumination units, and reference data corresponding to initial light amount distributions in a state with illumination of the illumination units, and stores them in a memory. The apparatus samples light amount distributions used to detect a light-shielded position, and calculates the light-shielded position based on three data, that is, the sampled light amount distributions, and the previously stored base data and reference data. By acquiring the base data and reference data upon power ON, and calculating a light-shielded position using these data, an effect of absorbing individual differences of CCD pixels that form light receiving units, and an effect of eliminating the influence of ambient light of an installation environment are obtained. In addition, an effect of absorbing a change in characteristic due to aging of the coordinate input apparatus, and an effect of eliminating the influence resulting from attachment of dust to the light receiving units due to aging are obtained.
However, during use after acquisition of the base data and reference data upon power ON, if this signal has changed due to the influence of some kind, the coordinate input apparatus malfunctions. For example, in an environment including ambient light such as an incandescent lamp, spotlight, or the like in the installation environment of the apparatus, if a light source as that ambient light is turned off or the position of the light source is changed during use, the current state becomes different from the environment upon power ON. That is, the environment upon power ON is different from that during use. For this reason, when a light-shielded position to be acquired during use is calculated using the base data and reference data acquired upon power ON, the detected position coordinates may include errors or the apparatus may malfunction.